1. © 2004 Pearson Addison-Wesley. All rights reserved April 10, 2006 Inheritance (part 2) ComS 207: Programming I (in Java) Iowa State University, SPRING 2006 Instructor: Alexander Stoytchev

2. © 2004 Pearson Addison-Wesley. All rights reserved Quick Review of Last Lecture

3. © 2004 Pearson Addison-Wesley. All rights reserved Parameter Passing (primitive types) The act of passing an argument takes a copy of a value and stores it in a local variable acessible only to the method which is being called. num138 Before: void myMethod(int num2) { num2 =50; } { int num1=38; myMethod(num1); } num138 After: num238 Before: num250 After:

4. © 2004 Pearson Addison-Wesley. All rights reserved Objects and Reference Variables acct1 72354 acctNumber 102.56 balance name “Ted Murphy” acct2 69713 acctNumber 40.00 balance name “Jane Smith”

5. © 2004 Pearson Addison-Wesley. All rights reserved Parameter Passing (objects) Objects (in this case arrays) are also passed by value. In this case, however, the value is the address of the object pointed to by the reference variable. void myMethod(int[] b) { b[0]+=5; } { int[] a={5, 7}; myMethod(a); } a Before: 5 7 a After: 10 7 b Before: 5 7 b After: 10 7

6. © 2004 Pearson Addison-Wesley. All rights reserved In the previous example there is only one array and two references to it. a 5 7 b

7. © 2004 Pearson Addison-Wesley. All rights reserved The array can be modified through either reference. a 10 7 b

8. © 2004 Pearson Addison-Wesley. All rights reserved Method Overloading The compiler determines which method is being invoked by analyzing the parameters float tryMe(int x) { return x +.375; } float tryMe(int x, float y) { return x*y; } result = tryMe(25, 4.32) Invocation

9. © 2004 Pearson Addison-Wesley. All rights reserved Method Overloading The compiler determines which method is being invoked by analyzing the parameters float tryMe(int x) { return x +.375; } float tryMe(int x, float y) { return x*y; } [signature 1] tryMe: int [signature 2] tryMe: int, float

10. © 2004 Pearson Addison-Wesley. All rights reserved Method Overloading The println method is overloaded: println (String s) println (int i) println (double d) and so on... The following lines invoke different versions of the println method: System.out.println ("The total is:"); System.out.println (total);

11. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance Inheritance is a fundamental object-oriented design technique used to create and organize reusable classes Here is a quick analogy

12. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance

13. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance

14. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance

15. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance

16. © 2004 Pearson Addison-Wesley. All rights reserved What can be inherited in Java?

17. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance In class hierarchies the Inheritance arrow usually points up instead of down.

18. Chapter 8 Sections 8.1 & 8.2

19. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance Inheritance allows a software developer to derive a new class from an existing one The existing class is called the parent class, or superclass, or base class The derived class is called the child class or subclass As the name implies, the child inherits characteristics of the parent That is, the child class inherits the methods and data defined by the parent class

20. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance Inheritance relationships are shown in a UML class diagram using a solid arrow with an unfilled triangular arrowhead pointing to the parent class Vehicle Car Proper inheritance creates an is-a relationship, meaning the child is a more specific version of the parent

21. © 2004 Pearson Addison-Wesley. All rights reserved Inheritance A programmer can tailor a derived class as needed by adding new variables or methods, or by modifying the inherited ones Software reuse is a fundamental benefit of inheritance By using existing software components to create new ones, we capitalize on all the effort that went into the design, implementation, and testing of the existing software

22. © 2004 Pearson Addison-Wesley. All rights reserved Deriving Subclasses In Java, we use the reserved word extends to establish an inheritance relationship class Car extends Vehicle { // class contents }

23. © 2004 Pearson Addison-Wesley. All rights reserved Book & Dictionary Example See Words.java (page 440) Words.java See Book.java (page 441) Book.java See Dictionary.java (page 442) Dictionary.java

24. © 2004 Pearson Addison-Wesley. All rights reserved Class Diagram for Words Book # pages : int + pageMessage() : void Dictionary - definitions : int + definitionMessage() : void Words + main (args : String[]) : void

25. © 2004 Pearson Addison-Wesley. All rights reserved The protected Modifier Visibility modifiers affect the way that class members can be used in a child class Variables and methods declared with private visibility cannot be referenced by name in a child class They can be referenced in the child class if they are declared with public visibility -- but public variables violate the principle of encapsulation There is a third visibility modifier that helps in inheritance situations: protected

26. © 2004 Pearson Addison-Wesley. All rights reserved The protected Modifier The protected modifier allows a child class to reference a variable or method directly in the child class It provides more encapsulation than public visibility, but is not as tightly encapsulated as private visibility A protected variable is visible to any class in the same package as the parent class The details of all Java modifiers are discussed in Appendix E Protected variables and methods can be shown with a # symbol preceding them in UML diagrams

27. © 2004 Pearson Addison-Wesley. All rights reserved Appendix E

28. © 2004 Pearson Addison-Wesley. All rights reserved

29. The super Reference Constructors are not inherited, even though they have public visibility Yet we often want to use the parent's constructor to set up the "parent's part" of the object The super reference can be used to refer to the parent class, and often is used to invoke the parent's constructor

30. © 2004 Pearson Addison-Wesley. All rights reserved The super Reference A child’s constructor is responsible for calling the parent’s constructor The first line of a child’s constructor should use the super reference to call the parent’s constructor The super reference can also be used to reference other variables and methods defined in the parent’s class

31. © 2004 Pearson Addison-Wesley. All rights reserved this

32. © 2004 Pearson Addison-Wesley. All rights reserved this super

33. © 2004 Pearson Addison-Wesley. All rights reserved this super this super

34. © 2004 Pearson Addison-Wesley. All rights reserved this super this super

35. © 2004 Pearson Addison-Wesley. All rights reserved Modified Book Example See Words2.java (page 445) Words2.java See Book2.java (page 446) Book2.java See Dictionary2.java (page 447) Dictionary2.java

36. © 2004 Pearson Addison-Wesley. All rights reserved Multiple Inheritance Car PickupTruck Truck (not allowed in Java)

37. © 2004 Pearson Addison-Wesley. All rights reserved Multiple Inheritance Java supports single inheritance, meaning that a derived class can have only one parent class Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents Collisions, such as the same variable name in two parents, have to be resolved Java does not support multiple inheritance In most cases, the use of interfaces gives us aspects of multiple inheritance without the overhead

38. © 2004 Pearson Addison-Wesley. All rights reserved Analogy

39. © 2004 Pearson Addison-Wesley. All rights reserved This example shows how multiple inheritance can be faked in java [http://www.vsj.co.uk/pix/articleimages/may05/javathread3.jpg]

40. © 2004 Pearson Addison-Wesley. All rights reserved THE END